Origin of Regiochemical Control in Rh(III)/Rh(V)-Catalyzed Reactions of Unsaturated Oximes and Alkenes to Form Pyrdines

Citations Over TimeTop 10% of 2019 papers

Abstract

The Rh(III)-catalyzed reactions of α,β-unsaturated oximes with alkenes are versatile methods for the synthesis of pyridines. Density functional theory (DFT) calculations reported here reveal the detailed mechanism and origins of selectivity in this reaction. The Rh(III)/Rh(V)/Rh(I) catalytic cycle was found to be more favorable than the previously proposed Rh(III)/Rh(I)/Rh(III) catalytic cycle. The Rh(III)/Rh(V)/Rh(I) catalytic cycle involves C–H activation, alkene insertion, deprotonation, oxime migratory oxidative addition, nitrene insertion, 1,5-hydrogen shift, and β-hydride elimination to give the pyridine product and form a Rh(I) species. Subsequent oxidation by Ag+ regenerates the Rh(III) catalyst. Reductive elimination from a alkyl-Rh(III) species is predicted to be difficult, so that the Rh(III)/Rh(I)/Rh(III) catalytic cycle can be excluded. The reactivities of oxime ethers and oxime esters are compared. The oxime ester acts as both a directing group and an internal oxidant. In this reaction, the N–O bond is activated by the pivalate, and migratory oxidative addition onto the Rh(III) species generates the corresponding Rh(V) nitrene complex. However, in the absence of the pivalate on the oxime ether, the activation energy for oxidative addition is much higher. The reactivity was analyzed by NPA charge calculations, comparison of the N–O bond orders, and the bond dissociation energies. The calculations also explain the regioselectivity of alkene insertion, which is shown to be an electronic effect rather than a steric effect.

Related Papers

• → A DFT Study of the Full Catalytic Cycle of the Suzuki−Miyaura Cross-Coupling on a Model System(2006)374 cited
• → Mechanism of Ni-Catalyzed Selective C−O Bond Activation in Cross-Coupling of Aryl Esters(2009)279 cited
• → Methanol Carbonylation Catalyzed by the Anion of the Complex Dicarbonyldiiodorhodium(I). A Density Functional Study of the Catalytic Cycle(2001)41 cited
• → Mechanism and Catalytic Impact of Ir–Ta Heterobimetallic and Ir–P Transition Metal/Main Group Interactions on Alkene Hydrogenation(2015)35 cited
• → Mechanisms of chemoselectivity for acyl and decarbonylative Suzuki–Miyaura coupling of N-acetyl amide with arylboronic acid catalyzed by Pd and Ni catalysts: Insights from DFT calculations(2020)10 cited